Here's an idea I've had a few times but never gotten around to implementing: when an application encounters a correctable data consistency error, or does something that's likely to fail, it should log something to a table so that another process (which may not be written until years later) can double-check that nothing messed up, or clean up after the operation. User (spammer?) deletion is a good one -- as in a large application, it's easy to forget to implement all the cleanup logic when adding new features.